Until you realise that there are any number of possible causes for gender dysphoria, like androgen receptor repetitions, or CYP17 genotype, or brain structure development, or BSTc region of the brain, or due to environmental personality conflicts, or any number of statistically linked and somewhat indirect causes. Are you really going to take all of these into account? Are you going to be able to find them all, to assign one to each case after a full genetic sequencing and months worth of testing and blood work and MRIs and whatever else? Are you going to be able to do that for everyone you run into on the street? No. You're not. Sex is not simple, it is not binary, and you have no scientific or medical basis on which to invade another person's life and start telling them how to live, how to think and how to feel.
Trying to force people into these convenient boxes isn't just needlessly hurtfull and unfair, it's impossible.